Steel Material for Machine Structural Use Having Excellent Contact Pressure Fatigue Strength

ABSTRACT

There is provided a steel material with superior surface pressure fatigue strength, comprising a machine structural steel used for power transmission components such as gears and shafts used in automobiles, industrial machines, and the like. The steel material comprises in mass %: C: 0.15 to 0.35%; Si: 0.30 to 0.95%; Mn: 0.10 to 1.00%; P: 0 to 0.030%; S: 0 to 0.030%; Cr: 0.80 to 2.30%; Cu: 0 to 0.30%; Al: 0.008 to 0.500%; O: 0 to 0.0030%; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.29%; Ti: 0 to 0.200%; Nb: 0 to 0.20%; and B: 0 to 0.0050%; and the balance Fe and unavoidable impurities. The steel material has, in mass %, a parameter represented by Si+Cr-2Mn of 1.05 or more and a parameter represented by 0.7Si+2.5Mn+2.0Cr+2.5Ni+4.0Mo of 6.30 or less.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2011-95390 filed on Apr. 21, 2011, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to machine structural steel materials, for example, used for power transmission components such as gears and shafts used in automobiles, industrial machines, and the like, and particularly relates to a machine structural steel material with superior surface pressure fatigue strength in the case of manufacturing a component by performing gas carburization.

BACKGROUND ART

When a usual JIS-standard steel material is used, grain boundary oxidation often occurs in an abnormally carburized layer formed on the surface of a steel component by gas carburization. The grain boundary oxidation is observed in wedge shape from the surface toward the inside and becomes a cause of decreasing surface pressure fatigue strength. Accordingly, improvement in surface pressure fatigue strength has been conventionally attempted by reducing grain boundary oxidation.

However, for general JIS SCr420 or SCM420, there has been proposed a technology to reduce the depth of an abnormally carburized layer by reducing the amount of Si and to further add alloy elements such as Cr and Mo to enhance temper softening resistance to improve surface pressure fatigue strength (e.g., see Patent Literature 1 (Japanese Patent Laid-Open Publication No. 2000-297347)). On the other hand, for a conventional JIS-standard case hardening steel, there has been proposed a steel improved in surface pressure fatigue strength by adding an increased amount of Si to enhance temper softening resistance and reducing the depth of an abnormally carburized layer (e.g., see Patent Literature 2 (Japanese Patent Laid-Open Publication No. 7-258793)). However, these Patent Literatures do not describe how the elements other than Si influence the depth and form of the abnormally carburized layer.

CITATION LIST Patent Literature

[PTL 1] Japanese Patent Laid-Open Publication No. 2000-297347

[PTL 2] Japanese Patent Laid-Open Publication No. 7-258793

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a steel material with superior surface pressure fatigue strength, comprising a machine structural steel used for power transmission components such as gears and shafts used in automobiles, industrial machines, and the like.

According to an embodiment of the present invention, there is provided a machine structural steel material with superior surface pressure fatigue strength, the steel material comprising in mass %:

-   -   C: 0.15 to 0.35%;     -   Si: 0.30 to 0.95%;     -   Mn: 0.10 to 1.00%;     -   P: 0 to 0.030%;     -   S: 0 to 0.030%;     -   Cr: 0.80 to 2.30%;     -   Cu: 0 to 0.30%;     -   Al: 0.008 to 0.500%;     -   O: 0 to 0.0030%;     -   N: 0.0020 to 0.0300%;     -   Ni: 0 to 3.00%;     -   Mo: 0 to 0.29%;     -   Ti: 0 to 0.200%;     -   Nb: 0 to 0.20%;     -   B: 0 to 0.0050%; and     -   the balance Fe and unavoidable impurities,     -   wherein the steel material has, in mass %, a parameter         represented by Si+Cr-2Mn of 1.05 or more and a parameter         represented by 0.7Si+2.5Mn+2.0Cr+2.5Ni+4.0Mo of 6.30 or less.

According to one preferred embodiment of the present invention, the above-described steel material may be substantially free of Ni, Mo, Ti, Nb, and B or may comprise them at unavoidable impurity levels, in mass %.

According to another preferred embodiment of the present invention, the above-described steel material may comprise, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.29%.

According to another preferred embodiment of the present invention, the above-described steel material may comprise, in mass %, at least one or more of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.29% and at least one or more of Ti: 0.020 to 0.200%, Nb: 0.02 to 0.20%, and B: 0.0003 to 0.0050%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view that illustrates the shape of a roller pitting test piece, in which numerical values are in mm.

FIG. 2 is a view that gives an explanation about carburizing and quenching and tempering examples by gas carburization, in which (a) and (b) illustrate the heating patterns of the carburizing and quenching, and the tempering, respectively.

DESCRIPTION OF EMBODIMENTS

The present invention is specifically explained below. In addition, % of each constituent element indicates mass %.

The machine structural steel material with superior surface pressure fatigue strength according to the present invention comprises in mass %: C: 0.15 to 0.35%; Si: 0.30 to 0.95%; Mn: 0.10 to 1.00%; P: 0 to 0.030%; S: 0 to 0.030%; Cr: 0.80 to 2.30%; Cu: 0 to 0.30%; Al: 0.008 to 0.500%; O: 0 to 0.0030%; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.29%; Ti: 0 to 0.200%; Nb: 0 to 0.20%; and B: 0 to 0.0050%; and the balance Fe and unavoidable impurities, preferably consists essentially of these elements and unavoidable impurities, and more preferably consists of these elements and unavoidable impurities.

The steel material according to the present invention comprises C in an amount of 0.15 to 0.35%, preferably 0.20 to 0.30%. C is an element necessary for securing the core strength of the steel material after carburizing, quenching and tempering thereof for a machine structural component. A content of C of less than 0.15% fails to secure the strength, while that of more than 0.35% results in reduction in toughness and increase in hardness to deteriorate workability.

The steel material according to the present invention comprises Si in an amount of 0.30 to 0.95%, preferably 0.40 to 0.85%. Si is an element that is necessary for deoxidation and that has the effect of imparting strength and hardenability necessary for steel and also the effect of reducing the depth of an abnormally carburized layer by addition thereof above a certain amount. The Si addition of 0.30% or more is necessary for obtaining this effect. On the other hand, an amount of added Si of more than 0.95% results in deteriorated workability because of increasing the hardness of a material.

The steel material according to the present invention comprises Mn in an amount of 0.10 to 1.00%, preferably 0.20 to 0.80%, more preferably 0.20 to 0.55%. Mn is an element necessary for securing hardenability. However, less than 0.10% of Mn fails to provide a sufficient effect for hardenability, while more than 1.00% thereof results in deteriorated mechanical workability.

The steel material according to the present invention comprises P in an amount of 0 to 0.030%, typically more than 0 and not more than 0.030%. P is an unavoidable optional element that is incorporated from scrap, but its content of more than 0.030% results in grain boundary segregation to deteriorate characteristics such as impact strength and bending strength.

The steel material according to the present invention comprises S in an amount of 0 to 0.030%, typically more than 0 and not more than 0.030%. S is an optional element that improves machinability, but its content of more than 0.030% generates MnS which is a non-metallic inclusion to deteriorate crosswise toughness and fatigue strength.

The steel material according to the present invention comprises Cr in an amount of 0.80 to 2.30%, preferably 1.10 to 2.15%. Cr is an element necessary for securing hardenability. However, less than 0.80% of Cr results in an insufficient effect for hardenability, while more than 2.30% thereof results in inhibited carburization and also in increased material hardness to deteriorate mechanical workability.

The steel material according to the present invention comprises Ni in an amount of 0 to 3.00%, preferably 0.20 to 3.00%. Ni is an optional element that has the action of enhancing hardenability and improving toughness to improve surface pressure fatigue strength. Addition of 0.20% or more thereof is preferred for obtaining this effect. The addition of 0.20% or more also influences the hardenability of a carburized layer. On the other hand, the Ni content of more than 3.00% significantly deteriorates workability and increases cost.

The steel material according to the present invention comprises Mo in an amount of 0 to 0.29%, preferably 0.05 to 0.29%. Mo is an optional element that improves surface pressure fatigue strength by the action of enhancing hardenability and enhancing the temper softening resistance of the steel material. Addition of 0.05% or more thereof is preferred for obtaining this effect.

The addition of 0.05% or more also influences the hardenability of a carburized layer. On the other hand, the Mo content of more than 0.29% deteriorates workability.

The steel material according to the present invention comprises Cu in an amount of 0 to 0.30%, typically more than 0 and not more than 0.30%. Cu is an unavoidable optional element that is incorporated from scrap, but has an aging property and is effective in increasing strength. However, the Cu content of more than 0.30% deteriorates hot workability.

The steel material according to the present invention comprises Al in an amount of 0.008 to 0.500%, preferably 0.014 to 0.300%. Al is an element that is used as a deoxidation material, and is bound to N to be precipitated as AlN to result in the effect of suppressing coarsening of grain size, as described below. Addition of 0.008% or more of Al is preferred for obtaining this effect. In contrast, addition of more than 0.500% of Al results in the formation of large-sized alumina-based inclusions and deteriorates fatigue characteristics and workability.

The steel material according to the present invention comprises B in an amount of 0 to 0.0050%, preferably 0.0003 to 0.0050%, more preferably 0.0010 to 0.0050%. B is an optional element that significantly improves the hardenability of steels, when contained even in a very small amount, and is effective in reducing the cost of the steel material since the amount of other alloy elements to be added can be reduced by the addition of B. Less than 0.0003% of B results in the small effect of improving hardenability, while more than 0.0050% thereof results in reduction in strength.

The steel material according to the present invention comprises O in an amount of 0 to 0.0030%, typically more than 0 and not more than 0.0030%, preferably not more than 0.0020%. 0 is an optional element that is unavoidably contained in steel. However, the 0 content of more than 0.0030% leads to the deterioration of workability and fatigue strength due to the increase of oxides.

The steel material according to the present invention comprises N in an amount of 0.0020 to 0.0300%, preferably 0.0020 to 0.0220%. N is finely precipitated as AlN and Nb nitrides in steel, and results in the effect of preventing coarsening of grain size, and addition of 0.0020% or more thereof is necessary for obtaining the effect. However, more than 0.0300% thereof increases the nitrides and deteriorates fatigue strength and workability. However, since Ti and N are bound to form hard TiN to conspicuously deteriorate mechanical workability when Ti is added, N is preferably regulated in a range of 0.0020 to 0.0100% and more preferably regulated in a range of 0.0020 to 0.0080%, particularly in a Ti-added steel.

The steel material according to the present invention comprises Ti in an amount of 0 to 0.200%, preferably 0.020 to 0.200%. Ti is an optional element that is bound to C in steel to finely form a carbide and results in the effect of preventing coarsening of grain size, and addition of 0.020% or more of Ti is necessary for obtaining the effect. On the other hand, since addition of more than 0.200% thereof deteriorates mechanical workability, the upper limit is 0.200%.

The steel material according to the present invention comprises Nb in an amount of 0 to 0.20%, preferably 0.02 to 0.20%, more preferably 0.02 to 0.12%. Nb is an optional element that forms a carbide or a nitride and results in the effect of preventing coarsening of grain size and, in particular, NbC or Nb (C, N) with a nanometer-order size, which is finely dispersed in steel, suppresses the growth of the grain size. Less than 0.02% of Nb fails to provide the effect, while more than 0.20% thereof results in the excessive amount of a precipitate to deteriorate workability.

The steel material according to the present invention has, in mass %, a parameter represented by Si+Cr-2Mn of 1.05 or more, preferably 1.05 to 2.30, more preferably 1.40 to 2.20. An abnormally carburized layer formed by gas carburizing and quenching is generated due to the deterioration of hardenability caused by the deficiency of the alloy elements in the surrounding area thereof. The deficiency is caused by the consumption of alloy elements due to the formation of oxides, in which oxygen is bound to Si, Mn, Cr, and the like, which are susceptible to oxidization, and the oxygen present in trace amount in a gas carburization atmosphere is supplied from a component surface. It has been considered that the abnormally carburized layer is normally associated with grain boundary oxidation formed during thermal exposure in gas carburization and therefore the grain boundary oxidation acts as a surface defect to reduce surface pressure fatigue strength. However, it has been found through extensive research by the present inventors that grain boundary oxidation which is a cause of reduction in surface pressure fatigue strength is reduced to form a dense abnormally carburized layer in the vicinity of a surface by controlling a parameter represented by Si+Cr-2Mn to reach 1.05 or more.

The steel material according to the present invention has, in mass %, a parameter represented by 0.7Si+2.5Mn+2.0Cr+2.5Ni+4.0Mo (parameter represented by 0.7Si+2.5Mn+2.0Cr in a steel material according to an embodiment comprising neither Ni nor Mo) of 6.30 or less, preferably 3.20 to 6.30, more preferably 3.80 to 5.80. By controlling this parameter to be 6.30 or less, hardenability in a carburized layer is kept low, and the hardness of an abnormally carburized layer can be softened. The dense and soft abnormally carburized layer can be interposed between the contact surfaces of components by satisfying both parameters for the density of the above-described abnormally carburized layer and the hardenability of the carburized layer. As a result, metal-to-metal contact between hard matrices can be avoided when each component is set to be fitted, and an appropriate contact surface is formed while the abnormally carburized layer moderately wears, whereby a good lubrication state can be achieved. The action of inhibiting fatigue cracking from proceeding is also obtained since the abnormally carburized layer is soft. By virtue of these effects, a machine structural steel with superior surface pressure fatigue strength can be obtained. Although shot peening or fine-particle shot peening for improving surface pressure fatigue strength can be omitted also by virtue of the superior effects, the surface pressure fatigue strength can be further improved by performing the shot peening or the fine-particle shot peening.

The present invention is the steel material comprising the steel having such steel constituents and parameters as described above; and by manufacturing power transmission components such as gears and shafts used in automobiles, industrial machines, and the like by performing gas carburizing and quenching and tempering using the steel material, components with high surface pressure fatigue strength in use for those applications can be provided.

EXAMPLES

The steel material according to the present invention is specifically explained with reference to Examples below.

100 kg of each of steels comprising the chemical constituents of the examples of the present invention steels and the examples of the comparative steels shown in Table 1 was ingotted in a vacuum melting furnace to obtain each of ingots. Subsequently, the ingots were heated to 1250° C. and maintained for 5 hours, followed by being forged to steel bars each having a diameter of 32 mm. Subsequently, the steel bars each having a diameter of 32 mm were heated to 900° C. and maintained for 1 hour, followed by being air-cooled and normalized. Then, the roller pitting test pieces 1, shown in FIG. 1, of these steels were produced and subjected to carburizing and quenching by gas carburization and tempering under the heating pattern conditions shown in FIG. 2. In addition, the comparative steel No. 29 was subjected to fine-particle shot peening after the gas carburizing and quenching and the tempering, while the comparative steel No. 30 was subjected to shot peening after the gas carburizing and quenching and the tempering.

TABLE 1

The shaded portions fall outside the scope of claims. Less than 0.20% Ni and less than 0.05% Mo are unavoidable impurities and are underlined. In addition, Fe is the balance and is omitted.

Then, these roller pitting test pieces 1 subjected to the carburizing and quenching and tempering treatment and further subjected to fine-particle shot peening or shot peening for comparison were subjected to a roller pitting test and the results are shown in Table 2. A roller pitting life as an index for surface pressure fatigue strength is shown in an intensity ratio on the assumption that the strength of the comparative steel 21 (equivalent to JIS SCM420) is 1.00.

TABLE 2 0.7Si + 2.5Mn + 0.7Si + 2.5Mn + 2.0Cr + No. Si + Cr − 2Mn 2.0Cr 2.5Ni + 4.0Mo Roller Pitting Life* Present Invention Steels 1 1.43 5.23 — 2.62 2 1.51 4.03 — 2.78 3 1.87 4.29 — 3.12 4 1.17 3.71 — 3.44 5 1.11 — 5.64 2.31 6 1.75 — 5.17 2.67 7 1.38 — 5.96 2.48 8 2.03 5.26 — 2.53 9 1.88 4.66 — 2.67 10 1.31 4.31 — 2.26 11 1.25 3.37 — 3.05 12 1.60 4.57 — 2.86 13 1.09 — 6.22 1.78 14 1.09 — 6.01 2.55 15 1.07 — 5.99 2.02 16 1.09 — 6.01 2.05 Comparative Steels 17 0.90 6.61 — 0.95 18 −1.13 4.46 — 0.83 19 0.09 4.60 — 0.75 20 0.01 — 7.29 1.20 21 −0.02 — 4.89 1.00 22 0.71 — 12.81  1.23 23 −0.5 — 3.30 0.95 24 0.61 — 4.85 1.09 25 0.03 — 4.29 0.73 26 −0.69 — 6.91 0.82 27 −0.23 — 7.34 1.13 28 −0.24 — 7.53 0.88 29 −0.39 4.13 — 1.73 30 0.05 — 7.03 1.65 *Shown in a ratio based on the roller pitting life of comparative steel No. 21(SCM420-equivalent steel) of 1.00.

As shown in Table 2, since each steel of the examples of the present invention comprises constituents in the specified range and has a parameter represented by Si+Cr-2Mn of 1.05 or more and a parameter represented by 0.7Si+2.5Mn+2.0Cr or 0.7Si+2.5Mn+2.0Cr+2.5Ni+4.0Mo of 6.3 or less, each of the steels of the examples of the present invention is greatly improved in roller pitting life in comparison with each steel of comparative steels Nos. 17 to 28. Each of the steels of the examples of the present invention also has a roller pitting life equivalent to or better than those of the steels of comparative examples No. 29 and No. 30 subjected to the shot peening or the fine-particle shot peening. 

1. A machine structural steel material with superior surface pressure fatigue strength, the steel material comprising in mass %: C: 0.15 to 0.35%; Si: 0.30 to 0.95%; Mn: 0.10 to 1.00%; P: 0 to 0.030%; S: 0 to 0.030%; Cr: 0.80 to 2.30%; Cu: 0 to 0.30%; Al: 0.008 to 0.500%; O: 0 to 0.0030%; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.29%; Ti: 0 to 0.200%; Nb: 0 to 0.20%; B: 0 to 0.0050%; and the balance Fe and unavoidable impurities, wherein the steel material has, in mass%, a parameter represented by Si+Cr-2Mn of 1.05 or more and a parameter represented by 0.7Si+2.5Mn+2.0Cr+2.5Ni+4.0Mo of 6.30 or less.
 2. The machine structural steel material according to claim 1, wherein the steel material consists of in mass %: C: 0.15 to 0.35%; Si: 0.30 to 0.95%; Mn: 0.10 to 1.00%; P: 0 to 0.030%; S: 0 to 0.030%; Cr: 0.80 to 2.30%; Cu: 0 to 0.30%; Al: 0.008 to 0.500%; O: 0 to 0.0030%; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.29%; Ti: 0 to 0.200%; Nb: 0 to 0.20%; and B: 0 to 0.0050%; and the balance Fe and unavoidable impurities.
 3. The machine structural steel material according to claim 1, wherein the steel material is substantially free of Ni, Mo, Ti, Nb, and B.
 4. The machine structural steel material according to claim 2, wherein the steel material is substantially free of Ni, Mo, Ti, Nb, and B.
 5. The machine structural steel material according to claim 1, wherein the steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.29%.
 6. The machine structural steel material according to claim 2, wherein the steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.29%.
 7. The machine structural steel according to claim 1, wherein the steel material comprises, in mass %, at least one or more of Ti: 0.020 to 0.200%, Nb: 0.02 to 0.20%, and B: 0.0003 to 0.0050%.
 8. The machine structural steel according to claim 2, wherein the steel material comprises, in mass %, at least one or more of Ti: 0.020 to 0.200%, Nb: 0.02 to 0.20%, and B: 0.0003 to 0.0050%.
 9. The machine structural steel material according to claim 1, comprising, in mass %, at least one or more of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.29% and at least one or more of Ti: 0.020 to 0.200%, Nb: 0.02 to 0.20%, and B: 0.0003 to 0.0050%.
 10. The machine structural steel material according to claim 2, comprising, in mass %, at least one or more of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.29% and at least one or more of Ti: 0.020 to 0.200%, Nb: 0.02 to 0.20%, and B: 0.0003 to 0.0050%. 